As an alternative for the computational waste generated by proof-of-work (PoW) blockchains, proof-of-stake (PoS) systems gained a lot of popularity, being adopted by many existing cryptocurrencies. Unfortunately, as we show, PoS-based currencies, where newly minted coins are assigned to the slot leader, inevitably incentivises coin hoarding, as players maximise their utility by holding their stakes and not trading. As a result, existing PoS-based cryptocurrencies do not mimic the properties of fiat currencies, but are rather regarded as investment vectors. In this work we initiate the study of minting mechanisms in cryptocurrencies as a primitive on its own right, and as a first step to a solution to mitigate coin hoarding in PoS currencies we propose a novel minting mechanism based on waiting-time first-price auctions. Our main technical tool is a protocol to run an auction over any blockchain. Moreover, our protocol is the first to securely implement an auction without requiring a semi-trusted party, i.e., where every miner in the network is a potential bidder. Our approach is generically applicable and we show that it is incentive-compatible with the underlying blockchain, i.e., the best strategy for a player is to behave honestly. Our proof-of-concept implementation shows that our system is efficient and scales to tens of thousands of bidders.